This invention relates to thin-film deposition systems, and more particularly to shadow masks to control the film deposition rate and uniformity.
Electrical and optical systems often rely on devices with thin films. For example, an optical filter may have a hundred layers of films stacked together, each being a quarter of a desired wavelength. These thin films must be deposited with a uniform thickness to prevent unwanted effects, such as a spreading of the wavelength filtered.
These thin films can be deposited in a vacuum chamber using a sputtering or ion beam method. FIG. 1 shows a thin-film deposition chamber using a shadow mask. Chamber 10 is part of a larger film-deposition machine. The air is pumped out of chamber 10 to produce a low-pressure, near-vacuum environment. A substrate 20 is placed in the chamber. One or more layers of thin films are deposited on the upper surface of substrate 20. Substrate 20 is rotated by motor 12 to improve uniformity of deposition.
Ion-beam source 14 generates ion beam 26 that is directed against target 18. Target 18 is made from an ultra-pure target material that supplies the atoms that form the film being deposited onto substrate 20. For example, target 18 can be of a silicon-dioxide SiO2 material or of Tantalum (Ta) material to adjust the index of refraction of the deposited film. Target 18 is a consumable item and must be replaced with a new target after films have been deposited.
When ion beam 26 impacts the surface of target 18, some of the target""s atoms are ejected or sputtered off of target 18. These atoms or ions from target 18 then travel from target 18 to substrate 20 as target beam 28. When the atoms from target beam 28 impact the upper surface of substrate 20, they attach to the surface and form a thin film. Over time, the film on substrate 20 becomes thicker and thicker until the desired thickness is reached, and ion-beam source 14 is turned off. Substrate 20 and target 18 can be heated to improve reaction rates.
As target 18 is consumed, and its surface roughness changes, so the rate that atoms or ions are sputtered off its surface can vary. To control the uniformity of film deposited on the substrate, shadow mask 24 can be used to partially block target beam 28. Shadow mask 24 may consist of several blades. Each blade is made especially for each of the different target materials. A technician can do a test run before each production deposition run to check the uniformity of deposition across the substrate and use the test run results to modify the shadow mask shape to achieve better uniformity. Motor 22 rotates shadow mask 24 to select one of the blades, or to move shadow mask 24 out of position so substrate 20 can be loaded or removed.
While the end of deposition of each layer of film can simply occur after a fixed amount of time, an optical monitoring system improves results. Light source 32 shines light beam 30 through substrate 20 and into endpoint detector 34. The thin-film deposited onto the surface of substrate 20 interferes with light beam 30, attenuating the intensity of light beam 30 before it reaches detector 34. This attenuation varies as the film becomes thicker during deposition. Endpoint detector 34 analyzes one or more wavelengths of light beam 30 and signals an endpoint when a predetermined attenuation is reached. Ion-beam source 14 is then turned off for that layer of film deposition. When non-transparent substrates are used, the light beam can be reflected off the surface of the substrate rather than transmitted through the substrate.
FIG. 2 shows a prior-art shadow mask. Substrate 20 is rotated at a predetermined speed so that the whole substrate 20 can be covered by target beam 28. Target beam 28 falls on target area 28xe2x80x2 which is where film deposition occurs. Variations of beam intensity within target area 28xe2x80x2 are thought to contribute to film non-uniformity.
Shadow mask 24 is a metal blade that obstructs the target beam where shadow mask 24 overlaps target area 28xe2x80x2. The blade can be shaped so that more obstruction occurs for smaller radii near the center of substrate 20 than for the larger radii near the outer rim of substrate 20. Areas near the center have a slower linear velocity than regions near the outer rim, and thus lower-radius areas spend more time inside target area 28xe2x80x2. To compensate, the shape of the blades reduces the film thickness near the center of substrate 20 relative to film thickness near the outer rim of substrate 20.
Shadow mask 24 can be rotated, allowing other blades 23, 25 to be selected to overlap target area 28xe2x80x2. Wider blades reduce the deposition rate, while thinner blades increase the deposition rate. All blades can also be rotated out of position so substrate 20 can be removed or loaded.
While using a shadow mask can improve film uniformity, the degree of control is limited since one of only three shadow mask blades can be pre-selected. The endpoint is determined by the light beam falling on area 30xe2x80x2 of substrate 20, which covers only a limited range of radii. Uniformity at different radii is typically checked by a technician who removes substrate 20 from the vacuum chamber and performs a series of measurements on the film. In-situ monitoring of the film at different radii is not provided, and no feedback control mechanism is used. The shadow mask is selected for use during the entire film deposition period and is not normally changed during a run.
Better uniformity of film thickness is desired. This can be achieved by using a multi-radius monitor that controls and varies the shadow mask within a deposition run.